Thin Solid Films 436(2003) 264–268

0040-6090/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0040-6090Ž03.00569-8

Improved host material design for phosphorescent guest–host systems

Travis Thoms *, Shinjiro Okada , Jian-Ping Chen , M. Furugoria, b a b

Canon Development Americas, Inc., 3300 North First St. Third Floor, San Jose, CA 95134, USAa

OL-Project CANON INC.5-1, Morinosato-Wakamiya, Atsugisi, Kanagawa, Japanb

Received 6 September 2002; received in revised form 19 February 2003; accepted 19 March 2003

Abstract

A series of proposed carbazole-based compounds are studied as host materials in an iridium phosphor-based guest–host organiclight-emitting diode. Semi-empirical calculations are performed on each compound to predict its efficacy as a host material, andthese theoretical predictions are compared to device performance for each compound, in an attempt to verify the model.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Optoelectronic devices; Iridium; Luminescence; Organic substances

1. Introduction

Organic light-emitting diodes(OLEDs) promise to bea revolution in light-emitting devicesw1,2x. With lowerpower usage and potentially easier fabrication methods,OLED devices could outperform liquid crystal displayor cathode ray tube technologies in personal digitalassistant displays, computer monitors, or television.Even as inorganic light-emitting diode(LED) devicescould replace incandescent or fluorescent light sources,OLEDs could replace inorganic LEDs in all aspects ofuse.

Currently, phosphorescent electroluminescent materi-als are a prime focus of OLED researchw3–5x. Theirtheoretical limit of efficiency makes them an attractivealternative to fluorescent counterparts, due to their abil-ity to use energy from both singlet and triplet excitedstates for light emission.

Unfortunately, there are few organic-based phospho-rescent materials that can be deposited as neat filmsw6x.Usually it is necessary to co-deposit them with a hostmaterial, either a charge transporting ‘small molecule’or polymer, to get a reasonable light output. This is duemainly to the formation of exiplexes in the material,which quench the desired emission.

Guest emitters can be doped into either hole-transportor electron-transport materials. Popular host materials,

*Corresponding author.E-mail address: sleditor@yahoo.com(T. Thoms).

such as hole-transporting 4,49-N,N9-dicarbazol-biphenyl(CBP) and electron-transporting aluminum 8-hydroxy-quinoline (AlQ ) w7x, are commercially available and3

have been used as a matter of convenience for manyguest–host applications. However, these may not havethermal nor electronic properties that allow for fulloptimization of the guest–host system. Thus, it is impor-tant to design either hole or electron-transporting mate-rials that have been optimized to support a given emitter,or family of emitters, to ensure maximum efficiencyand color purity from the phosphorescent guest. Unfor-tunately, using synthetic methods and derivatization toexplore host design for a guest chromophore can beexpensive and time-consuming. This paper offers a fairlyreliable method of designing and selecting host materialsfor a given system using computational methods.

2. Experimental details

2.1. Computational methods

Computationalw8x work investigating compounds1–9and CBP(Fig. 1) was done on an IBM PC platformusing WINDOWS 2000 . A 3D computer construct of�

each host compound was made. The geometry of themolecule was optimized twice to find the lowest groundstate energy conformation, and the energy levels of themolecular orbitals were determined, from which wasextracted an estimated first triplet excited state(T1).

265T. Thoms et al. / Thin Solid Films 436 (2003) 264–268

Fig. 1. Molecular structures of Compounds1–9 and CBP.

The details for this procedure: structures were drawnand a preliminary geometry optimization of each mole-cule (using the MM2w9x engine, default settings) wasperformed using Hyperchem 6.0(a product of Hyper-�

cube, inc.). The structure files were converted and afinal geometry optimization was completed usingMOPAC 6.0 w10x and the AM1 w11x semi-empiricalhamiltonian (Restricted Hartree-Fock method anddefault settings). Structures were then converted backto Hyperchem format and single point CI determinationswere made via the ZINDOyS w12x method(RestrictedHartree-Fock method, Polack-Ribiere conjugate gradientoptimization and default settings) to determine theoret-ical T1 excited-state energy levels.

2.2. Materials

Compounds1–9 and CBP are synthesized usingUllman coupling reactionsw13x. Bathocuproine(BCP),iridium tris(phenylpyridine) (IRPPy3), N,N9-bis-(1-nap-thyl)-N,N9-1-diphenyl-1,19-biphenyl-4,49-diamine (a-NPB)), iridium tris(phenylpyridine) (IrPPy3) andALQ were purchased from Dojindo Laboratories.3

2.3. HOMO–LUMO level estimations

All compounds listed were measured using a cyclo-voltametric apparatus with platinum control and working

electrodes, and a silverysilver nitrate reference electrode.Thin films of each material were cast on the workingelectrode, and measurements were made in a solutionof 0.1 M TBABF4 in acetonitrile. LUMO levels wereestimated by adding the optical band-gap measured fromUV–Vis absorption measurements to the calculatedHOMO.

2.4. Fluorescence and UV absorption measurements

Measurements for fluorescence were made on a PTIspectrofluorometer. Compounds were dissolved in meth-ylene chloride and excited at their peak excitationwavelength. The UV–Vis absorption spectrum shown,as well as those used to calculate optical band-gap, wasmeasured on a Varian Cary 50 Conc spectrophotometer,using a dilute solution in methylene chloride.

2.5. Triplet excited state measurements

Measurements were made using photoluminescenceon a Hitachi F-4500 spectrofluorometer. Compoundswere dissolved in a 4:1:5 methanolyethanolytoluenesolution and cooled to 77 K, photoemission was thenmeasured, using a chopper to remove shorter-lived sin-glet emissions. Peak phosphorescent wavelengths werethen converted to electron-volts for publication.

2.6. Device fabrication

All of organic layers were deposited by thermalevaporation via an ULVAC thermal deposition chamberonto indium tin oxide at 10 Pa. A layer ofa-NPBy4

was deposited to 40 nm, followed by deposition of the40 nm thick emitting layer consisting of IrPPy3 dopedinto the host material(5 wt.%), an exciton-blockinglayer of 10 nm thick BCP, and an electron-transportinglayer of 400 nm thick AlQ . A 10 nm layer of alumi-3

num–lithium alloy(AlLi ) (Li 1.8 wt.%) covered by 150nm of aluminum was deposited on it as a cathode.

3. Results and discussion

In development of an effective guest–host system,there are two major concerns: Aggregation, andguest–host energy level and orbital alignment. Efficientelectroluminescence in a guest–host system is dependenton the method of excitation of the guest; this beingeither charge or energy transfer. With charge transfer,where electrons and holes are transported through theguest–host matrix leading to charge recombination onthe guest, it is preferred that the band-gap of the guestfalls within the band-gap of the host(Fig. 2). In asystem where energy transfer is the predominant mech-anism, recombination occurs on the host molecule, andthe energy created is transferred from the excited stateof the host to the excited state of the guest.

266 T. Thoms et al. / Thin Solid Films 436 (2003) 264–268

Fig. 2. Preferred relative HOMO–LUMO energy levels of guest andhost materials.

Fig. 3. Comparison of guest–host T1 energy level alignment betweensuboptimal and optimal guest–host material matches in phosphores-cent systems.

Table 1Summary of calculated and experimentally determined T1 energy lev-els of proposed host materials

Experimental Computational

T1 (peak) T1 host T1 T1 host(eV) –T1 CBP(peak) (eV) –T1 CBP

CBP 2.67 0.00 0.01 0.001 2.38 y0.29 y0.39 y0.402 2.66 y0.01 y0.34 y0.353 2.37 y0.31 y0.39 y0.404 2.53 y0.15 y0.22 y0.235 2.63 y0.04 y0.32 y0.336 2.82 0.15 0.10 0.097 3.02 0.35 0.06 0.058 3.02 0.35 0.14 0.139 2.82 0.14 0.11 0.10

In an energy-transfer based guest–host system wherelight emission is from the relaxation of a singlet excitedstate, it is important to promote efficient energy transferbetween the singlet-excited states of the guest and thehost. An easy way of verifying this overlap is to comparethe emission spectrum of the host to the absorptionspectrum of the guestw14x. If there is good overlap,there is a good assumption of efficient energy transferfrom the host to the guest. Such an approximation doesnot hold when dealing with singlet-hostytriplet-guestconditions, such as the studied carbazole hostyIrPPy3system. If the exited triplet state of the guest is higherthan that of the host, good energy transfer will notoccur, even with good overlap of host emission andguest absorption(Fig. 3). This holds true for charge-transfer systems as well, since a lower T1 level in ahost could allow energy to be transferred from theexcited guest molecule to the host after charge recom-bination. By designing hosts with a higher first excitedstate triplet state(T1), the phosphorescent guest willlose less of its energy through non-radiative means.

Since charge transfer is the more likely mechanismfor IrPPy3ycarbazole-based hostsw15x, the host designershould make certain that the band-gap of the guest fallscomfortably within the band-gap of the host, and thenconcern himself with the alignment of the triplet excitedstates between the two materials.

The focus of this work is the computational calcula-tion of the first excited state triplet energy levels of aseries of host candidates, using semi-empirical methods.These data are then compared with the calculated T1level of a host known to be effective for the desiredguest. If the T1 level of the compound in question fallsbelow that of the known host, its efficacy as a hostmaterial is doubted.

There are two important caveats to this method. First,all hosts to be considered should have a good deal ofchemical similarity. The less similar the compounds, theless accurate the semi-empirical methods of determining

relative energy levels are. Second, this approach assumesthat the T1 level of the standard host is the lowestenergy that can still effectively transfer to the guestmaterial(or prevent guest to host energy transfer). Sincethis may not be immediately the case, the model tendsto be iterative; as new host materials with lower T1levels than the known host are proven to be effective,they then become the standard for comparison.

Table 1 tabulates the energy levels of compounds1–9and CBP; these levels both experimentally measuredand calculated using the ZINDOyS method. The ZIN-DOyS method was selected from the number of semi-empirical methods available due to its higher rate ofrelative accuracy in preliminary calculations during thisstudy. Compounds having a predicted T1 greater thanCBP (whereDT1 is positive) should prevent guest-to-host triplet state energy transfer. The experimental datashows that the model is correct in its relative positioningof the T1 excited states of the series compared to CBP.Table 2 verifies their effectiveness as hosts through theirincorporation into a device. All compounds predicted tohave a higher T1 than CPB have a maximum efficiencyof greater than 0.5 lmyW, while those that are not sopredicted have considerably less.

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Table 2Comparison of device performance vs. expected results of proposedhosts

Compound Predicted host Max. experimental Model predictsEfficacy (lmyW) Experiment

CBP Standard 6.21 Poor -0.3 TRUE2 Poor -0.3 TRUE3 Poor 0.01 TRUE4 Poor 0.004 TRUE5 Poor 0.083 TRUE6 Good 12.2 TRUE7 Good 0.7 TRUE8 Good 0.8 TRUE9 Good 1.8 TRUE

Table 3HOMO–LUMO measurements of IrPPy3, CBP and compounds1–9

Compounds HOMO LUMO(eV) (eV)

1 y5.6 y2.42 y5.5 y2.33 y5.8 y2.54 y5.6 y2.45 y5.7 y2.26 y5.7 y2.37 y5.5 y2.08 y5.6 y2.19 y5.6 y2.1CBP y5.5 y2.1Ir(PPy)3 y5.2 y3.2

Fig. 4. (a) Fluorescence spectra of the hosts and(b) IrPPy3 lightabsorption spectrum.

It is worthwhile to note that all of these test com-pounds have roughly the same fluorescent emission(Fig. 4) and that these wavelengths fall within theabsorption band of the guest. It is also noteworthy thatall of the host band-gaps are easily large enough tocontain the band-gap of the guest(Table 3). If this wasa singlet-emission guest, all of the tested compoundsmay have been suitable as hosts for this guest–hostmatrix. This reiterates the importance of predicting theT1 levels of any host material used for a phosphorescentguest.

Returning to Table 2, we can see that, of the foursuccessful hosts,7 and8 did the poorest. Crystallizationand thermal breakdown were noted during device fab-rication and testing using these two compounds. This isdue to their inherent thermal instability easily predictable

from their size and shape. Compounds6 and 9 remainas the hosts that show equal or greater promise to CBP,and are worthy of further study and development.

To summarize, in order to design a new host for ouriridium phosphor, we could have suggested compounds1–9 as suitable replacements for CBP. The computationalmodel would have eliminated all candidates, except for6 through9. From these remaining four, one could havesurmised that compounds6 and 9 would have the bestthermal properties. Now, instead of synthesizing all ninecompounds for trial, we have efficiently eliminated theplausible candidates down to two.

4. Conclusion

As phosphorescent materials become even more pre-dominant in the field of OLEDs, suitable host materialswill need to be designed and improved upon in thecoming years. Simple semi-empirical calculation meth-ods, like the one described, can increase the rate atwhich these hosts are efficiently and economicallydesigned, by comparing a chemically similar set of hostcandidates to that of a known host, and eliminatingthose compounds least likely to be effective. The model,along with simple application of various rules of thumb,can drastically reduce the amount of synthetic workneeded to develop efficient host materials, bringing evengreater efficiency from phosphor-based OLED devices.

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